1. Field of the Invention
The present invention relates generally to gas turbine engines and, more particularly, to a turbine nozzle support arrangement.
2. Description of the Prior Art
Gas turbine engines typically include a core engine having a compressor for compressing air entering the core engine, a combustor where fuel is mixed with the compressed air and then burned to create a high energy gas stream, and a first or high pressure turbine which extracts energy from the gas stream to drive the compressor. In aircraft turbofan engines, a second turbine or low pressure turbine located downstream from the core engine extracts more energy from the gas stream for driving a fan. The fan provides the main propulsive thrust generated by the engine.
An annular high pressure nozzle is located between the combustor and high pressure turbine and between stages of the turbine. The annular nozzle includes a pair of radially spaced inner and outer annular bands disposed concentrically about a longitudinal axis of the core engine and a plurality of airfoils supported between the inner and outer annular bands. The airfoils are arranged in circumferentially spaced relation from one another and extend in radial relation to the core engine axis. Either the inner band or the outer band may include some form of flange for coupling the nozzle to a stationary engine support structure. The annular nozzle is provided by a plurality of arcuate segments which fit end-to-end together to form the 360.degree. circumferentially extending nozzle. Each nozzle segment includes arcuate segments of the inner and outer bands and a pair of the airfoils mounted side-by-side between the inner and outer band segments.
The annular high pressure nozzle provides the function of directing and/or re-directing hot gas flow from the combustor into a more efficient direction for impinging on and effecting rotation of the rotor stages of the high pressure turbine. The directing process performed by the nozzle also accelerates gas flow resulting in a static pressure reduction between inlet and outlet planes and high pressure loading of the nozzle. Also, the annular nozzle experiences high thermal gradients from the hot combustion gases and the coolant air at the radial support surfaces.
In common nozzle support systems, the nozzle segments are attached by bolted joints or a combination of bolts and some form of clamping arrangement to an engine support structure. In some stages, such as the first stage nozzle, the nozzle segments are attached by bolted joints to the engine stationary support structure via a radially inner mount or flange structure coupled to the inner band segments. The radially outer band segments are not mechanically retained but are supported against axial forces by a circumferential engine flange. In other stages, such as stage 2 of an engine, the nozzle segments may be attached at their radially outer band segments but be free at its radially inner band segments.
In either design, the use of bolted joints and clamps at spaced circumferential locations about the segments of a nozzle band act as restrictions to free thermal expansion of the band. Due to the band being hotter than the support structure to which it is attached, or due to thermal gradients within the bands, radial or axial bowing of the band segments of the nozzle occurs which, in turn, produces leaking of gas flow from the flow path or leaking of cooling air into the flow path and stressing of bolts, support flanges and the airfoils attached to the band, leading to crack formation.
Alternative mounting arrangements have been proposed to eliminate the use of bolts and clamps. In one alternative nozzle mounting arrangement the segments of the nozzle are mounted on two pins per segment. The pins are retained in blind holes in the engine support structure. This design allows the nozzle segment to rock axially on the support structure via a chordal hinge defined in the flange of the nozzle segment. One of the two mounting pins per segment makes a tight fit in the nozzle flange to position the nozzle segment accurately in the tangential and radial directions. The other of the two mounting pins makes a loose fit in the nozzle flange. However, the axial rocking of the nozzle segment causes bending stress in the tight-fitting pin in the same way as a bolted joint described above. Pin wear is also a problem in this design.
In another alternative nozzle mounting arrangement, the nozzle segment is also allowed to rock axially on a chordal hinge. The nozzle segment is retained axially and tangentially by radially-extending bolts through the nozzle support structure. These radial bolts do not attach to the nozzle segment, but are retention features only. The nozzle segment is positioned in the engine by the gas loads which positively locate the nozzle segments axially against the nozzle support flange and tangentially against the radial bolts. While this arrangement allows the nozzle segment to rock axially without transmitting bending stress to the bolts or nozzle support structure, its main disadvantage is that leakage areas are created by the axial bowing of the nozzle segments due to thermal gradients.
In yet another alternative nozzle mounting arrangement proposed by the patent application cross-referenced above, interfitting hooks and studs and overlying projections and lands are provided on the outer band segments of adjacent nozzle segments and on adjacent portions of the support structure. These mounting features substantially overcome the disadvantages of bolted or clamped nozzles by providing a positive attachment between the nozzle and an adjacent engine support structure. However, a need still remains for development of alternative designs which will provide further improvements in mounting the nozzle segments to the engine support structure.